The present invention relates to a circuit for detecting unbalance of the traction current in a track circuit of the type comprising a track section which is short-circuited by rolling stock in such a manner as to form an independent track circuit, each such section being provided with a transmitter for transmitting a track circuit current and an associated receiver. The invention extends to a circuit arrangement for detecting the presence of rolling stock on a track section, comprising the detecting circuit, the transmitter and the receiver, and also to a section of railway track connected to such a circuit arrangement.
The traction current for locomotives equipped with electronic control has a harmonic content which under determined conditions is similar to the actual signal of the track circuit.
The traction current and the actual current of the track circuit both pass simultaneously through the track circuit, by means of which data is transmitted. Under conditions of substantial unbalance between the two rails, due for example to accidental grounding or fracture of a rail, or to disconnection of the continuity braid or strip between two rails, the traction current can give rise to potential differences across the track circuit receiver which is equivalent to the signal of the track circuit, and thus cause undue excitation of the relays in the receiver or blocking section.
One condition of the track circuit which would cause unreliability is the presence of the train on a track in which there is a rail fracture at a point behind the train. Under such a condition, the traction current, with a high harmonic content generated by the rolling stock or substations and in the presence of beats between the various frequencies, is associated with one half of the receiver (inductive connection in this case), and can therefore produce across the receiver a voltage equivalent to the track circuit current. This can cause the undue excitation of the track relay controlling the signal in the block section occupied, consequently setting the signal at "go" (green) whereas it should remain at "stop" (red) for obvious safety reasons.